Methinks there are possession rules for demons in the Book of Vile Darkness if you have that. I don't have the rules memorized, and I don't know if I'd be allowed to post them if I did since they're closed content, but in short, the possessee gets a saving throw against the possession (will save I think). If he succeeds, he can't be possessed for 24 hours. A possessed creature can then be possessed in a few different ways. The possessor can ride the host (host doesn't know) in order to get past wards and such, help the host (give a bonus to a stat), hinder the host (give a penalty to a stat), or control the host. When controlling the host, the host gets regular saving throws (can't remember if it's every round or every hour or what).
In short, a DM can really ruin your day with possession.